Discovery may be "Smoking Gun" in Gamma Ray Burst Mystery

Discovery may be "Smoking Gun" in Gamma
Ray Burst Mystery

UAH Scientist at NASA/Marshall Leads International
Team of Discovery

March 31, 1997An international team of University
and NASA astronomers have detected a flash of light from a distant
galaxy - a sentinel that may definitively solve a 30-year mystery
in astrophysics by demonstrating that cosmic gamma-ray bursts
come from the distant reaches of the universe.

That would mean gamma-ray bursts represent an energy release
in a few seconds equivalent to the amount our Sun will emit in
its entire ten-billion-year lifetime.

A team led by Dr.
Jan van Paradijs, the Pei-Ling Chan eminent scholar in physics
at The University of Alabama in
Huntsville and a professor of astronomy at the University
of Amsterdam, has identified a dim, distant galaxy as the possible
source of a gamma-ray burst that swept through the sky on February
28.

His team includes Dr.
Chryssa Kouveliotou of the Universities Space Research Association
at Marshall, Dr. Richard Strom at Westerbork Radio Observatory
in the Netherlands, and graduate students Paul Groot and Titus
Galama at the University of Amsterdam.

The detailed results of their reseach have been accepted for
publication in the scientific journal, Nature.
These findings could force many astrophysicists to rethink their
theories about the origins of these violent explosions of energy.

One of the least understood phenomena
in astrophysics, gamma-ray bursts are powerful flashes of gamma
rays that are detected about once a day from random locations
in the sky. Gamma rays are electromagnetic radiation at the highest
end of the energy spectrum, carrying energies almost a million
times higher than visible light.

When they happen, gamma-ray bursts outshine all other sources
of gamma rays combined.

The discovery of gamma-ray bursts in the late 1960s was accidental.
Since that time, astronomers who tried to find the source of gamma-ray
bursts found nothing unusual when they looked in the directions
from which bursts originated.

Competing theories, developed using data from
NASA's orbiting Burst
and Transient Source Experiment (BATSE), place the bursts'
origins either just outside the Milky Way galaxy in a large spherical
halo, or from remote parts of the universe at what astronomers
call "cosmological distances."

On Feb. 28, the Gamma-Ray Burst Monitor aboard the Italian-Dutch
BeppoSAX satellite
detected a gamma-ray burst that was also within the field of view
of one of the SAX Wide Field Cameras. Built by the Netherlands
Space Research Organization (SRON), these cameras can pinpoint
in the sky the position of a burst within a circle roughly one-fifth
the diameter of the Moon as seen from Earth.

Gamma-ray burst detectors aboard the Ulysses and Wind spacecraft
also saw the burst.

"With the location narrowed to that small area, we were
able to obtain a few optical images of that region of the sky
with the 4.2 meter William Herschel telescope of the La Palma
Observatory in the Canary Islands," van Paradijs said. "We
took our first image just 21 hours after the burst was detected,
and the second about a week later. We were astounded by what we
saw."

In the first image, van Paradijs and his team found a light
source that does not appear on the second, apparently a much-sought-after
and almost legendary "optical transient."

Within eight hours after the burst was detected, the BeppoSAX
spacecraft was maneuvered to point its more precise X-ray imaging
instruments at the location indicated by the Wide Field Camera.
The X-ray detectors saw a glowing X-ray source, 5,000 times fainter
than the Crab Nebula. Its position was narrowed to within one
arc minute (one-sixth the diameter of the original box), consistent
with the source of the gamma-ray burst.

Three days later, the X-ray source was virtually gone, dropping
in brightness by nearly a factor of 20.

"The X-ray results by themselves are already very exciting,"
said van Paradijs. "But now that we also have an optical
transient, we can nail its position to better than one arc second
(one sixtyeth of the one-arc-minute box)."

"What is most interesting,"
said Kouveliotou, "is that an image taken March 13 by the
New Technology Telescope at the European Southern Observatory
in Chile found, in nearly the exact spot where the optical transient
appeared in the first photograph, a very faint galaxy."

The locations of the optical transient and the galaxy are
separated by at most one-quarter of an arc second - the width
of a quarter viewed from a distance of more than 10 miles.

"To me, this is fairly convincing evidence that the transient
X-ray and optical sources are the same, and that both are associated
with the gamma-ray burst," said van Paradijs. "If these
transients are from the distant galaxy, we have, for the first
time, found the site of a gamma-ray burst."

Astronomers do not know exactly how far away the faint galaxy
is, but they do know it is distant. The Hubble Space Telescope
has been pressed into duty to take a look.

"Hubble should have about a one hundred-or-so pixel elements
from which to make an image, so we should get a very nice picture,"
said van Paradijs.

Check out the
Latest Hubble Public-Domain Images of the GRB Transient Region

The Wide-field Planetary Camera Image of the Region in the
Visual Band. The Optical Transient is at the center of the photograph.
Click on image for larger view.

A Close-Up of the Optical Transient shows both a point-like
source (the bright emission) plus the extended emission (below
and to the right) from what may be the distant host-galaxy. Click
on image for larger view.

Note: Another follow-up observation is scheduled for April
7, 1997.

"Most researchers in the field, based largely on the BATSE
data, were already leaning toward the cosmological hypothesis,"
said Dr. Charles
Meegan, a BATSE co-investigator and gamma-ray burst expert
at Marshall. "But this detection might even convince the
remaining skeptics."

Even van Paradijs, however, is not totally
convinced that the distant galaxy was home to the Feb. 28 gamma-ray
burst: "We'd like to see it happen a few more times to be
sure," he said. "But right now, it absolutely has the
right smell."

"A couple more would be nice," agreed Dr.
Gerald Fishman, principal investigator on BATSE. "BATSE
is more sensitive to gamma-ray bursts than BeppoSAX, and it offers
an opportunity to confirm this phenomenon on other bursts.

"We've begun an intensive international, multi-spacecraft
campaign, in collaboration with ground-based observatories around
the world, to provide the locations of BATSE-detected bursts so
that others can do real-time follow up. It's a much better mousetrap
than we had before."

"The discovery of an optical flash and the possible host
galaxy for a gamma-ray burst marks a major advance in our quest
to understand this enigmatic phenomenon," said Dr.
John Horack of the Space Sciences Laboratory at Marshall.
"This may eliminate the possibility that gamma-ray bursts
originate from a halo around our galaxy."

Scientists who say bursts come from cosmological distances
must develop theories for much more powerful explosions than those
needed to explain bursts if they come from a halo of neutron stars
surrounding our galaxy.

Scientists can measure how powerful gamma-ray bursts are when
they reach Earth, but that tells them nothing about the distance
the gamma rays have travelled. (With no information about a radiation
or light source, for instance, you cannot see a difference between
a 100-watt bulb one mile away and a 400-watt bulb two miles away.)
Bursts from distant galaxies would have to travel billions of
light years farther than bursts from "nearby" neutron
stars.

Independent of what the distance is, however, most experts
agree that each source of a gamma-ray burst must undergo a powerful
explosion, in which its material is ejected at velocities approaching
the speed of light.